The Milky Way, or at least a portion of it, is older than
scientists assumed. According to a recent research, a portion of the disk is
two billion years older than previously assumed.
Only 0.8 billion years after the Big Bang, an area known as
the thick disk began to emerge.
Astronomers have put together the Milky Way's history in
more detail than ever before. Their findings are based on data from the Gaia
mission of the European Space Agency and China's Large Sky Area Multi-Object
Fiber Spectroscopic Telescope (LAMOST). Subgiant stars hold the key to this
finding.
"A time-resolved picture of our Milky Way's early
creation history," says the report, which is published in the journal
Nature. Maosheng Xiang and Hans-Walter Rix, both of the Max-Planck Institute
for Astronomy, are the authors (MPIA.)
The age of a star is one of the most difficult things to
ascertain. The composition, or metallicity, of a star is crucial in determining
its age. The more precisely astronomers can detect metallicity, the more
precisely they can establish the age of the object.
The early Universe was almost entirely made up of hydrogen
and helium. When stars die and burst, heavier elements than hydrogen and helium
are generated and spread out into the Universe. Every element heavier than the
two fundamental elements is referred to as a "metal" by astronomers.
Lower metallicity stars are older because they developed
when hydrogen and helium were the only elements available. As a result,
astronomers can tell that a population of stars mostly made up of hydrogen and
helium is older. When scientists identify a population of stars with greater
metal compositions, they know the stars are younger.
In certain areas of astronomy, such as this one, precision
age determinations are the holy grail. To establish stellar ages, Xiang and Rix
employed more than simply metallicity. They concentrated on one sort of star in
particular: subgiants.
Because the subgiant phase of a star's existence is so
brief, astronomers can most precisely establish a star's age while it is a
subgiant. Subgiants are transforming into red giants, and their cores are no
longer producing energy. Fusion has instead migrated to a shell around the
core.
The scientists utilized LAMOST data to assess the
metallicity of around 250,000 stars in various sections of the Milky Way in
their research. They also used Gaia data, which provides accurate location and
brightness information for 1.5 billion stars.
The Gaia mission of the European Space Agency is responsible
for improved accuracy in this and many other studies. Prior to Gaia,
astronomers had to deal with star age uncertainties ranging from 20% to 40%.
This meant that ages may be inaccurate by a billion years, which is a
significant amount of time.
Gaia, on the other hand, has transformed everything. Gaia
EDR 3, or Early Data Release 3, is the mission's most recent data release, and
it's a huge improvement. Over 330,000 stars are precisely located in 3D using
EDR3. It also provides extremely accurate measurements of the movements of the
stars across space.
The researchers applied all of the data from Gaia and LAMOST
to existing stellar parameter models to establish the subgiants' ages with
higher precision. "We can calculate the age of a subgiant star to a few
percent using Gaia's brightness data," Maosheng added.
A merger two billion years later brought the star creation
in the dense disk to a close. Gaia-Sausage-Enceladus, a dwarf galaxy, merged
with the Milky Way.
The dwarf galaxy Gaia-Sausage-Enceladus (GSE) isn't shaped
like a sausage. It receives its name from the fact that its stars' orbits are
unusually elongated when shown on a velocity chart. When GSE joined with the
Milky Way, it contributed to the creation of the thick disk, and the gas that
accompanied it spurred star formation in that region of the galaxy.
The merger also added stars to the Milky Way's halo. The
remaining core of the Gaia Sausage is thought to be the globular cluster NGC
2808. NGC 2808 is one of the Milky Way's most enormous globular clusters.
The GSE initiated star production in the dense disk that
lasted around 4 billion years. The gas was depleted about 6 billion years after
the Big Bang. During that time, the metallicity of the thick disk grew by a
factor of 10.
The investigation also discovered a strong link between the
metallicity of the stars and their ages over the whole disk. That means the gas
that arrived with the GSE had to be turbulent, which caused it to mix more
completely in the disk.
The GSE merger was very recently found by astronomers in 2018.
Such discoveries have impacted our view of the Milky Way's past, and the
galaxy's evolutionary timetable is becoming clearer. This new study provides a
more in-depth description.
"Astronomers have hypothesized that the Milky Way was
already there before the halo formed since the finding of the ancient merger
with Gaia-Sausage-Enceladus in 2018, but we didn't have a good image of what
that Milky Way looked like," Maosheng explains.
"Our findings reveal a wealth of information about that
region of the Milky Way, including its age, star formation rate, and metal
enrichment history. Putting these findings together with Gaia data is
completely changing our understanding of when and how our galaxy was
created."
Astronomers have just uncovered fresh information about the
Milky Way. However, because we're in the middle of it, mapping its structure is
difficult. The Gaia mission of the European Space Agency is the most
comprehensive inventory of stars in the Milky Way yet. And each data release
improves on the previous one.
"Gaia allows us to put together the history of our
galaxy in unparalleled detail with each new research and data release.
Astronomers will be able to add even more details to the tale when Gaia DR3 is
released in June "Timo Prusti, ESA's Gaia Project Scientist, agrees.
Although the Gaia mission is critical, studies of other
galaxies, such as the Milky Way, provide scientists with information on the
Milky Way's structure and history. However, two billion years after the Big
Bang, spotting galaxies is challenging. This necessitates the use of large
infrared telescopes. Thankfully, one long-awaited infrared space telescope will
shortly begin observations.
The James Webb Space Telescope (JWST) has the ability to see
back into the early years of the Universe. It will be able to observe the
oldest Milky Way-like galaxies in the Universe.
Astronomers are interested in learning more about the GSE
merger and how it produced our galaxy's broad disk barely two billion years
after the Big Bang. JWST studies of old, high-redshift galaxies akin to the
Milky Way should help answer certain issues and fill in some gaps in our understanding
of the galaxy's past.
The ESA will also release Gaia's entire third data release,
dubbed DR3, in June. Over 7 million stars' ages, metallicities, and spectra
will be included in the DR3 collection. The JWST and DR3 will be a powerful
combo.
What will all of this information reveal? Galaxies must
either eat or be consumed as the Universe develops. Galaxies are drawn together
by gravity, but the Universe is expanding due to dark energy, which pushes
galaxies away. As a result, galaxies prefer to form groupings. The Milky Way is
a member of the Local Group of galaxies.
The galaxies' combined gravity keeps the groupings
internally cohesive, but expansion causes them to drift apart from one another.
The larger galaxies in a group eventually eat the smaller ones.
The GSE and globular clusters have been swallowed by the
Milky Way. And it's devouring the Large Magellanic Cloud, which is devouring
the Small Magellanic Cloud, which is devouring the Large Magellanic Cloud.
The Milky Way will eventually engulf both, and then join
with the far bigger Andromeda Galaxy, another member of the Local Group, in
roughly 4.5 billion years.
It's a peculiar circumstance since the Milky Way's future
may be easier to predict than its history. That's the problem with an expanding
Universe: the proof we seek keeps vanishing, lost to time and distance.
The JWST and the Gaia DR3, on the other hand, have the ability to turn the tide against the expanding Universe. They can provide more information on the Milky Way's history and the specifics of galaxy mergers in general if they work together. Hopefully, we'll be able to compile a much more comprehensive historical chronology.
